1. Field of the Invention
The invention concerns a magnetic resonance device of the type having a cavity and with a gradient coil system arranged in the cavity.
2. Description of the Prior Art
Magnetic resonance technology is a known technology for, among other things, acquiring images of the inside of a body of an examination subject. In a magnetic resonance device, rapidly switched gradient fields that are generated by a gradient coil system are superimposed on a static basic magnetic field that is generated by a basic field magnet. The magnetic resonance device also has a radio-frequency system that emits radio-frequency signals into the examination subject to excite magnetic resonance signals and acquires the excited magnetic resonance signals, on the basis of which magnetic resonance images are generated.
To generate gradient fields, appropriate currents must be adjusted in gradient coils of the gradient coil system. The amplitudes of the required currents can be up to more than 100 A. The current rise and fall rates can be more than 100 kA/s. The existing basic magnetic field, on the order of 1 T, interacts with these temporally changing currents in the gradient coil to produce Lorentz forces, which leads to oscillations of the gradient coil system. These oscillations are transmitted over various propagation paths at the surface of the magnetic resonance device. The mechanical oscillations are thereby transduced into sound vibrations that subsequently lead to undesired noise. Furthermore, the Lorentz forces can lead to an undesired rigid-body motion (resonance) of the gradient coil system with regard to the rest of the magnetic resonance device.
A magnetic resonance device is known from German OS 197 22 481 in which a basic field magnet has a first surface and a gradient coil system has a second surface, the surfaces facing one another being separated from one another, and a noise reduction device is arranged in contact with both surfaces to damp the oscillations of the gradient coil system and/or to stiffen or reinforce the gradient coil system. In an embodiment, to form a closed, sealed space between the two surfaces, the noise reduction device has suitable seals, this space being filled with sand, foam, a fluid under pressure, or other oscillation-damping and/or stiffening materials. In another embodiment, the noise reduction device has a number of cushions that can be filled with one of the aforementioned materials. In another embodiment, in a basic field magnet having a cylindrical, hollow opening, in which a hollow-cylindrical gradient coil system is arranged, the noise reduction device is formed by wedges that are distributed between the two surfaces.
A magnetic resonance device with a gradient coil system is known from German OS 101 56 770, in which an electrically conductive structure is arranged and fashioned such that a magnetic field of the structure caused by a gradient field by induction is similar to the gradient field, at least within the imaging volume of the magnetic resonance device. In an embodiment, a part of the structure is fashioned substantially barrel-shaped as a component of the basic field magnet. Among other things, the gradient coil system can be fashioned without shielding coils, since the undesired consequences of the switched gradient fields, due to the similarity of the magnetic field caused by the structure, can be completely controlled by a pre-emphasis (pre-distortion/deformation).